Lines and Symmetry — Answer Key
Part A: Fill in the Blank
Write the missing word or number on each line.
1. Two intersecting lines that do not form right angles are called oblique lines.
Oblique lines cross each other but do not make 90-degree angles. If they did form right angles, they would be called perpendicular instead.
2. A regular triangle has 3 line(s) of symmetry.
A regular triangle (equilateral) has all sides the same length, so you can fold from each of its 3 vertices to the opposite side's midpoint and the halves match every time.
3. The symbol → is used to show a ray in math notation.
The arrow symbol → represents a ray because a ray starts at a fixed point and continues forever in one direction, just like an arrow that keeps going.
4. An isosceles trapezoid has 1 line(s) of symmetry.
An isosceles trapezoid has two equal-length legs, so the only fold that makes both halves match is a vertical line through the middle of its two parallel sides.
5. The capital letter I has 2 line(s) of symmetry.
The capital letter I can be folded both vertically and horizontally through its center and the halves match each way, giving it 2 lines of symmetry.
6. A rectangle has 2 pair(s) of parallel sides.
A rectangle has 2 pairs of parallel sides: the top and bottom are parallel to each other, and the left and right sides are parallel to each other.
7. Two rays that share the same endpoint form an angle.
When two rays start from the same point, the opening between them is called an angle. The shared starting point is the vertex of the angle.
8. A regular decagon has 10 lines of symmetry.
For any regular polygon, the number of lines of symmetry equals the number of sides. A decagon has 10 sides, so it has 10 lines of symmetry.
9. If you fold a shape along a line of symmetry, the two halves overlap perfectly.
A line of symmetry divides a shape into two identical halves. When you fold along that line, every point on one side lands exactly on the matching point of the other side, so the halves overlap perfectly.
Part B: Matching
Match each item on the left to the correct answer on the right.
1. Match each item to its correct answer.
Regular pentagon
→ 5 lines of symmetry
6 lines of symmetry
Isosceles triangle
→ 1 line of symmetry
0 lines of symmetry
Regular hexagon
→ 6 lines of symmetry
1 line of symmetry
Right scalene triangle
→ 0 lines of symmetry
5 lines of symmetry
Correct matches: Regular pentagon → 5 lines of symmetry; Isosceles triangle → 1 line of symmetry; Regular hexagon → 6 lines of symmetry; Right scalene triangle → 0 lines of symmetry.